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FLOX burners turn mega

In many high-temperature processes, so-called FLOX burners provide optimum fuel utilisation with low pollutant emissions. Flameless oxidation – known as FLOX – has already become established in the middle output range between 20 and 300 kilowatts. Scientists are now working as part of the Mega-FLOX project to expand the output range. This opens up new areas of application.

With experimental studies and computer simulations, researchers at WS Wärmeprozesstechnik and the RWTH Aachen have developed methods and instruments which allow burner sizes to be scaled. For the tests, the researchers constructed a FLOX burner in the megawatt range, which they were able to develop to series production level with just a small number of modifications. However, this not only allows large megawatt burners to be developed, but also small burners, for example for micro gas turbines. According to Joachim Wünning, project leader and managing director at WS Wärmeprozesstechnik, the decisive advantages are not only the low nitrogen oxide emissions: “The FLOX process is particularly flexible when it comes to the composition of the fuel gas. It is suitable for synthetic gases, residual gases and lean gases. That’s why the burners also offer advantages with applications where there are no nitrogen oxide problems at all, such as in places where convention burners fail to function perfectly.”

This opens up countless application opportunities. Until now, FLOX burners have been used in many industrial processes, mainly in the area of the thermal treatment of metals, such as in annealing, hardening or heat-treatment equipment. With a higher output in the megawatt range, FLOX burners also have a good chance of being used in reformers in the chemical industry, or in large heating and processing furnaces. With power-to-gas plants, FLOX burners are also suited to utilising lean gases which otherwise would not be used.

Low in pollutants despite high temperatures

Industrial furnaces usually operate at very high process temperatures. Accordingly, the exhaust gases from the burners also leave the furnace chamber at a high temperature. These energy losses can be minimised with recuperator burners, which use the exhaust gases to pre-heat the combustion air. However, at higher combustion air temperatures, the formation of nitrogen oxides also increases. The nitrogen oxide emissions alone represent a significant challenge in high temperature processes. Often, the threshold values defined in the German Technical Instructions on Air Quality Control (TA Luft) can only be achieved with costly downstream purification of the exhaust gases.

Atmospheric nitrogen oxidises to a notable extent in the hot zones of the flame front. By contrast, with FLOX burners, almost no nitrogen oxides are produced, even though they also pre-heat the combustion air with the exhaust gas. The decisive difference compared to conventional recuperator burners is the flameless oxidation of the burner gas – without the temperature peaks of a flame front. This leads to a consistent furnace temperature, which can be maintained at up to around 1,400 degrees Celsius with lower nitrogen emissions.

Flameless combustion can be achieved through a sophisticated mixture of fuel gas, combustion air and recirculating exhaust gas. Fuel gas and combustion air flow into the combustion chamber with a higher flow speed. Strong internal exhaust gas circulation mixes the air and exhaust gas in the combustion chamber, and then mixes this mixture with the fuel gas after a delay. As a result, flameless oxidation occurs above a temperature of around 650 degrees Celsius. Accordingly, this leads to very homogeneous temperatures throughout the entire furnace volume. Nitrogen oxides, which occur particularly on flame fronts with high peak temperatures, are hardly created at all. The thorough mixing ensures complete combustion, so that the level of carbon monoxide concentration in the exhaust air is also negligible.

FLOX – how it all began

Attempts by WS Wärmeprozesstechnik to create a recuperator burner, which began at the end of 1989, led to a surprising observation: at furnace temperatures of 1,000 degrees Celsius, and with air preheating of 650 degrees Celsius, the monitoring device for the burner flame ceased to indicate a signal, and no flame could be heard. Nevertheless, the fuel was being burnt completely. The carbon monoxide content of the exhaust gas was below 1 ppm, and the NOx emissions were so low, that it was initially assumed that the measuring device was malfunctioning. However, the combustion was stable and uniform, without a flame having formed. The scientists called this phenomenon flameless oxidation (FLOX).